Step 1. Take off your watch from the wrist. Put your Apple Watch face down to a soft and thick cloth or mat.

Step 2. Press and hold the "band release button" on the back of your apple watch in oval near the watch top and bottom parts. Simply slide the strap to the left and slide out, repeat for the other side of the strap.

Step 3. Make sure the outside of the new band towards outwards, slide each side of the band into slots until you hear a click into place.

Video guide on how to replace Apple Watch Bands from Apple

Hope more easier to follow step by step, and pls watch the Apple official watch band change video below: